I. Field of the Invention
The present invention relates generally to multiparty communication systems, and, more particularly, to a point-to-multipoint private communication network directly incorporated within a cellular or land line telephone system.
II. Description of the Related Art
Mobile cellular telephone service has been in use for some time, and traditionally has been characterized by a central site transmitting with high power to a limited number of mobile or portable units in a large geographic area. In early cellular systems only a limited number of radio channels were available, thus limiting the number of radiotelephone conversations within an entire metropolitan area to the number of channels available.
Modern cellular radiotelephone systems have a comparatively large number of radio channels, which may be effectively multiplied by using identical channel frequencies within the differing smaller coverage areas (i.e., "cells") comprising a given service territory. Each cell includes a cell-site transmitter, or base station, which broadcasts at a power level selected to ensure signal reception at the cell boundary without unduly interfering with reception in adjacent cells. This allows channel frequencies used in one cell to be reused in another cell geographically separated therefrom according to a predetermined plan. Thus, a large number of channels can be made available in a metropolitan area and the service provided thereby can be identical to a standard wire line telephone.
Numerous standards exist for the implementation of cellular telephone communications. These standards include the advanced mobile phone system (AMPS), Global System for Mobile communication (GSM), and Code Division Multiple Access (CDMA). The spread spectrum modulation technique of CDMA has significant advantages over other modulation techniques for multiple access communication systems. For example, the use of CDMA results in a much higher spectral efficiency than can be achieved using other multiple access schemes.
Although recent development efforts have enabled CDMA and other cellular systems to effectively provide "point-to-point" communication links between users, various public and private agencies have nonetheless continued to rely upon dedicated land mobile radio (LMR) communication networks. This results from the incapability of cellular systems to establish "point-to-multipoint" communication networks among a set of member users. For example, local law-enforcement agencies utilize LMR networks in which a closed radio communication system is established through repeater stations. Such closed LMR networks are often characterized by push-to-talk (PTT) operation, in which users depress a handset talk button or the like when desiring to broadcast voice information to other member users. However, the infeasibility of providing repeater stations over a large geographic area limits the extent to which the set of member users may be geographically dispersed.
Although both cellular and conventional land line telephone systems are capable of facilitating communication between widely separated users, closed "PTT-type" communication networks have not hitherto been incorporated within either type of system. This may be due in part to the absence of a convenient mechanism for automatically joining an identified set of users into such a closed network. Moreover, even if such a mechanism were available, both types of systems are easily compromised by unauthorized third parties and are thus unsuitable for secure communication.
The conference calling capability provided by both cellular and land line carriers is also an unsuitable surrogate for a PTT-type communication network. In particular, conference calling between users within different cellular or land line systems requires some degree of prior coordination with the responsible service provider. Moreover, in many conference calling systems the information signals from the conference call participant are combined and the resultant composite signal universally provided to each such participant. This effectively precludes separately encrypting each such information signal as a means of increasing communication security, since the separately encrypted information signals would generally not be recoverable from the composite signal.
Accordingly, it is an object of the present invention to incorporate a private communication network directly within a cellular and/or land line telephone system in such a way as to obviate the need for prior coordination with a telephone service provider.
It is another object of the invention that the private communication network emulate an LMR network characterized by PTT operation.
It is yet another object of the present invention that control of the private communication network be resident within a network call manager separately connected to an existing land line telephone system.
It is a further object of the present invention that encryption techniques be capable of application within the private communication network as a means of enhancing communication security.